Abstract
BACKGROUND: Osmylusfulvicephalus (Scopoli, 1763), the type species of the Osmylidae, has garnered significant research interest in taxonomy, particularly regarding its larval stages. To date, most studies have focused on the third instar larvae, but ignore the first instar larvae with the rough and incomplete morphological descriptions. This lack of studies has led to an unclear understanding of the morphological differences between larval stages, making it challenging to identify larvae, distinguish different osmylid species or even reconstruct phylogeny. NEW INFORMATION: In this study, the first instar larva of O.fulvicephalus was re-described in detail and illustrated, based on the naming system of sclerites. Based on this, it could be effectively distinguished from other species by the following characteristics: i) number of segments in the labial palpi; ii) the setae beard on the apex of antenna; iii) the number of transversal rows of the anal hooks. In addition, we compared the sclerites and other morphological characteristics with the third instar larvae of Osmylusfulvicephalus to better distinguish larvae at different developmental stages.